The present invention relates to lamp phosphors, and particularly to coated phosphor powders exhibiting increased lumen maintenance when used in such lamps, as well as coating methods therefor.
Fluorescent lamps are known to experience a gradual decrease in brightness with extended operation of the lamp. There are several factors contributing to this low lumen maintenance including deposits of impurities from the lamp cathode onto the lamp envelope, the formation of various mercury compounds due to the bombardment of the phosphor by atoms and ions of the mercury included in the envelope, changes in the phosphor itself through inter- or intra-atomic reactions, and changes in the glass of the envelope, e.g., due to exposure to ultraviolet radiation. One of the chief causes of low lumen maintenance is the deposition of mercury compounds on the surface of the phosphor coating the interior surface of the lamp envelope. Such deposition is particularly deleterious in such lower cost, low maintenance phosphors such as zinc orthosilicate (synthetic willemite) phosphors.
Attempts have been made to coat phosphor particles with, e.g., a coating of an inert material such as alumina or other oxide, before their use in the lamp making process, to improve lumen maintenance in the lamp. It is particularly important that such a coating be highly conformal, since a conformal coating provides continuous coverage of each phosphor particle, shielding the phosphor from, e.g., chemical attack during lamp processing and operation. However, some of these coating attempts have resulted in poor adherence of the coating to the phosphor, or in non-conformality of the coating to the phosphor particle after, or even before bake-out of the lamp. Also, solid state diffusion of the oxide components into the bulk phosphor material at high temperatures can generate a new phase boundary between the oxide layer and the phosphor substrate. Undesirable phases can result, e.g., a phase having a high concentration of dislocations or defects in its crystal structure. Such excess dislocations or defects can act as energy traps for the phosphor under certain conditions of solid state diffusion thermodynamics, lowering the lumen output of the phosphor and defeating the purpose of the coating.
Further, many prior art processes for applying the oxide coatings to the phosphor particles involve suspending the phosphor powder in a fluidized bed for MOCVD (metal organic chemical vapor deposition) of the coating. This MOCVD process requires the use of a fluidized bed reactor, elaborate instrumentation, careful air flow and temperature monitoring, and separating out of a very narrow phosphor particle size range for successful suspension of the powder in the fluidized bed. In most of these processes, the oxide coating is deposited on a fully fired phosphor substrate, requiring an extra process step to apply the coating and increasing the cost of the phosphor fabrication process.
Non-CVD processes for applying protective coatings to phosphor particles using colloidal silica or precipitation of barium phosphate provide cost effective protection of the phosphor particles. These processes, however, provide only physically adsorbed coatings which require careful monitoring of the process to prevent spotty or non-conformal coatings.
The coating of individual phosphor particles with organometallic compounds, i.e., metal alkoxides, and converting the alkoxides to protective metal compounds during lamp burn-out is described in U.S. Pat. No. 5,196,229 to Chung N. Chau, one of inventors of the present invention. U.S. Pat. No. 5,196,229 is incorporated by reference herein. This method produces continuous, non-particulate, conformal, chemically adherent coatings on phosphors, and results in improved lumen maintenance in lamps in which the coated phosphor is used. However, the method is restricted in its use by the limited choice of precursor materials.
Accordingly, it is an object of the present invention to provide a phosphor and a phosphor coating method which overcome the disadvantages of the prior art.
It is yet another object of the invention to provide a phosphor with improved maintenance properties.
It is still another object of the invention to provide a coated phosphor exhibiting modified surface properties.
It is a further object of the invention to provide a coated phosphor exhibiting improved coating adherence, continuity, and conformality in the powder and in the finished lamp.
It is yet a further object of the invention to provide a reliable, cost effective chemical adsorption process for production of a coated phosphor exhibiting improved maintenance properties.